Vanes are commonly arranged in a circumferential array spanning an annulus through which a working fluid is directed. A vane typically comprises an aerofoil with a pressure side wall and a suction side wall which meet one another at a leading edge and a trailing edge. The pressure and suction side walls border a central chamber into which cooling air may be delivered through a radially directed inlet. An array of outlets is commonly provided adjacent the trailing edge of the vane, typically on the pressure surface side. The outlets are directed to deliver spent cooling air into the main working fluid flow which is directed over the pressure side wall and suction side wall.
The provision of air systems, in particular cooling-air systems for gas turbine engines, is known. Often, such systems when provided for cooling purposes in the hot section of the engine are dimensioned or designed such that they give adequate cooling under the most adverse conditions, for example at maximum power and the associated highest temperature ranges. Commonly, the source of cooling air is air taken off from the compressor.
During different operating conditions, the temperature of the hot section varies and can often be below the highest experienced. Consequently, non-adaptive cooling systems can result in excessive air mass flow and overcooling under operating conditions requiring lesser cooling. A larger air mass than is needed is supplied to the turbine and subsequently exhausted. Where the cooling air is sourced from the compressor, the efficiency of the engine is compromised leading to increased specific fuel consumption and in the case of aircraft powered by such engines, the range of the aircraft may be reduced.
Prior published U.S. Pat. No. 6,779,967B2 seeks to address the described limitations by introducing adaptive valve control. An embodiment described in U.S. Pat. No. 6,779,967B2 is reproduced in FIG. 1.
FIG. 1 shows, in highly simplified representation, a side view of a partial area of an aircraft gas turbine. A combustion chamber is here indicated by the reference numeral 7. A downstream turbine features a turbine casing 8 within which stator vanes 9 of a first stage and rotor blades 10 of the first stage are shown. The rotor blades 10 are attached to a rotor disk 11 of the first stage in conventional manner. Further in the downstream direction, a stator vane 12 of a second stage is shown which is associated with a rotor blade 13 of the second stage, this rotor blade 13 again being attached to a rotor disk 14 of the second stage. Reference numeral 15 indicates a turbine exit guide vane.
FIG. 1 further shows, in highly simplified form, a piston-cylinder unit 4 which is a part of an embodiment of the device for air mass flow control according to the present disclosure. The piston-cylinder unit 4 is located in the area of an inlet duct 1, exposed to a cooling air flow, with flow in the inlet duct issuing into an air duct 2 branching off from the inlet duct 1. The inlet duct 1 and air duct 2 may, for example, be used for ducting air from the cooling air flow to cool the stator vanes 9 or 12, respectively.
In the downstream direction, a counter-pressure duct 3 is provided by which pressure, for instance, from the turbine section of the engine, is applied to the rearward area of the piston 5 of the piston-cylinder unit 4. Furthermore, the piston-cylinder unit 4 comprises a spring 16 by which a suitable pre-load is applied to the piston 5 to bias the piston in the desired direction.
During operations with a high pressure difference, the pressure force in the inlet duct 1 exceeds the sum of the pressure force in the counter-pressure duct and of the pre-load force of the spring 16. The piston 5 is accordingly displaced such that the flow area of the air duct 2 is cleared. During operations with a low pressure difference, the pressure force in the counter-pressure duct 3, together with the pre-load force applied by the spring 16, exceeds the pressure force in the inlet duct 1, with the effect that the piston 5 is displaced to partly cover the free cross-section of the air duct 2, thus reducing the supply of air.
The present disclosure seeks further to make efficient use of air flows for the purposes of cooling vanes.